Infection of cells with virions of the mutant polyoma ts3 and SV40-ts 101 is temperature sensitive, whereas infection with naked DNA is not. The maintenance of the transformed phenotype in cells transformed by these viruses is also temperature sensitive. The bypass of temperature sensitivity during infection with DNA alone, combined with the failure of these mutants to complement with other virus mutants, has suggested the existence of a mutant virion protein. The potential existence of an identifiable and characterizable mutant virion protein which is responsible for the altered growth regulation in transformed cells makes it imperative to study and characterize the virus particles of these two mutants.